Legumes and nutrient management improve phosphorus and potassium balances in long‐term crop rotations

Balancing P and K inputs with crop needs is challenging in cropping systems applying poultry litter (PL) for N. We compared P and K balances of PL‐amended organic and mineral‐fertilized conventional systems over 13 yr. In organic systems, lower legume cover crop biomass (3,462 ± 421 vs. 4,691 ± 436 kg ha −1 ) and N contributions in a corn ( Zea mays L.)–soybean [ Glycine max (L.) Merr.] rotation (Org2) led to 58% greater PL applications to corn than in a corn–soybean–wheat ( Triticum aestivum L.) rotation (Org3); as did alfalfa ( Medicago sativa L.) in a corn–soybean–wheat–alfalfa rotation (Org6). Greater Org2 and Org3 P inputs reduced harvest exports by 106 and 102 kg ha −1 , respectively, increased mean P balances by 209 kg ha −1 , and mean Mehlich 3 P concentrations by 9.6 mg kg −1  compared to Org6. Alfalfa exported 62% of P and 56% of K applied throughout the Org6 rotation. Due to alfalfa export, the annual P surplus was small (9 kg ha −1 ) and soil P was “optimum” in Org6. Soil test‐based fertilizer application in tilled and no‐till conventional systems with initially high soil P led to a –124 kg ha −1 P balance and optimum soil P. Fertilizer and PL K applications exceeding crop uptake led to positive K balances in all systems but greater retention of PL than fertilizer K. Legume cover crops or forages and increasing crop rotation length/complexity improved organic system P and K balances. Soil test‐based fertilizer application in conventional systems reduced high soil test P while maintaining yields.